The invention relates generally to oscillators and more particularly to oscillators providing a substantially continuous output signal that tracks the frequency and phase of a repetitive non-continuous input signal.
One particular environment in which such as oscillator is useful is in color television apparatus where a color subcarrier signal is required that follows the frequency and phase of the color burst signal, which is typically about eight cycles of a signal at the color subcarrier frequency repeating at the television signal horizontal line rate.
In so-called "color correctors" in color television recording and reproducing systems (video tape and disc recorders, for example), the raw, unprocessed reproduced or "off-tape" composite color television signal has an unstable chroma portion that causes visible color errors in the produced picture if left uncorrected. The degree of instability is greatest in less expensive video tape recorders in which no capstan servo control system is employed.
In highly sophisticated video tape recorder systems, elaborate techniques are used to stabilize the reproduced video signal. For example, time base correction using both analog and digital techniques are well-known. Also velocity compensation is well-known is sophisticated systems for correcting errors that change through a single horizontal television line.
Color correctors, on the other hand, have been used in less sophisticated video tape recorders or for playback monitor purposes in the sophisticated machines where greater phase error and phase jitter can be tolerated in a trade-off against the cost of full color correction by time base correction and velocity compensation.
A typical color corrector uses the demodulation-remodulation technique: the chroma portion of the off-tape video signal is demodulated into its R-Y and B-Y components using a continuous subcarrier derived from the off-tape color burst using a phase lock loop. The demodulated R-Y and B-Y components are applied to a modulator having a stable subcarrier signal reference, as from a crystal oscillator, to provide a stabilized chroma signal.
One problem with such a color corrector using a phase lock loop is that the loop speed cannot be optimized to follow the fast phase changes of the off-tape color burst. Consequently, substantial phase errors are uncorrected in the remodulated chroma signal and line-by-line correction is not achieved.
Another approach has been to use a start-stop oscillator in place of the phase lock loop in the color corrector to achieve greater speed. Although initial phasing is achieved, the oscillator frequency wanders by the end of the line, causing hue shifts in the reproduced picture.